Electrodeposition of chromium



Patented Oct. 8, 1935' PATENT OFFICE auras-11 smc'raonsrosrnon or cnaomuu Oskar Krlmer, Leipzig Germany, alllgnor to Theodor Haebler, New York, N. Y.

No Drawing. Application May 81, 1982, Serial No. 614,631

'lClaims.

This invention relates to the electrodeposition of chromium from chromic acid in an aqueous solution.

In previously known chromium platingbaths,

the practically possible deposition of chromium metal ends with a presence of catalyst in excess of from 1.7 to 2 parts in 100 parts of chromic acid in a given quantity of solution; that is, with a catalyst ratio in excess of from 1.7 to 2.

In the solution of the present invention, however, the catalyst ratio may be greater than 2. Apparently the optimum ratio is 2.5 and the maximum 5-at current densities of from '54 to l ampere per square inch. and with temperatures from 15 degrees C. to 40 degrees C.

Also, according to the present invention, an agent is employed in the bath for combatting the evil effect of large accumulations of iron ions in the bath. While said agent includes a plurality of elements in chemical combination for acting as a radical to combine with iron ions in the bath to form desirable iron compounds, another agent is employed in the bath to protect such radical from becoming chemically dissociated.

I have so far obtained best results with a bath composed as follows (per litre of solution):

' Grams CrOa (chromic acid, with 25% S04) 500 804 (as in H2804) 12.5 KC4H506 (potassium tartrate) 2.5 KCN (potassium of cyanide) 2.5

The purpose of the cyanide or equivalent content of the bath is explained as follows: The bath during electrodeposition of chromium, accumulates iron ions, such ions becoming troublesome upon accumulating to the extent of about grams per litre of solution in highly concentrated chromic acid solutions, such as those having 400 to- 500 grams of chromic acidper litre. The cyanide radical content of the bath of the present invention, however, combines with iron ions to form a new iron cyanide compound. These cyanide ions should be stabilized, as otherwise, undesirable effects may occur.

Such behaviour of the cyanide radicalcontent of the bath, by reaction thereof with oxygen, is circumvented, according to the present invention, by the addition of an agent acting to eliminate such an oxidation of the cyanide as will generate the gaseous compounds just above referred to. As aforesaid, I prefer to employ the tartaric substance of KCiHsOa for such stabilization. The

action of such a substance is as follows: The acid radical thereof (0411500) or (0411406) comblues with the oxygen anions to form formic acid (HCOOI-I) Thereby, these anions are prevented from exercising their great aiilnity for the individual elements of the carbon-nitrogen radical of cyanide, to break up said radical to form NO: and

C0:. As a consequence, the CN is maintained to form, with iron ions in the bath, RON ferric cyanide; and to form, with the iron ions and the K in the bath, while the current is off, K:Fe(CN) e, potassium ferric cyanide. 10

Regarding the iron contents which is permissible in the bath it should be noted that a certain iron content not exceeding an upper limit of about 20 grams per litre may be even advantageous in increasing the current yield and the 15 like. To a bath which is altogether free from iron I may even add some soluble iron salts, or I may use the bath in a preparatory electrolysis with iron anodes so as to electrolytically dissolve some iron therein. 20

In such a case the agent capable of precipitating the iron should not be added at once, but only after such a period of operation, that the bath contains an excess of iron above the limit mentioned above. 25 A similar beneficial influence is exerted by small quantities of other metals of the iron group, for instance nickel and cobalt, which should be used in a chemically equivalent proportion. In-

mium-nickel or chromium-cobalt salts. These 35- salts permit an increased sulfuric acid content of the bath.

Various changes may be made in the details disclosed in the foregoing specification without departing fromv the invention or sacrificing the 40 advantages thereof.

In the claims afllxed to this specification no selection of any particular modification of the invention is intended to the exclusion of other modifications thereof and the right to subse- 4 quently make claim to any modification not covered by these claims is expressly reserved.

I claim:

' l. A chromic acid bath for electrodeposition of chromium having in solution during electrolysis, chromic acid and more than two per cent. of sulphate radical or equivalent based on the amount of chromic acid in solution, and also approximately one-half of one per cent. of KCN and one-half per cent. of KCAHBOB- l5 2.. The methodof conducting the electrod iting of chromium from solutions of. chromic acid, which comprises establishing a bath oi. chromig acid, sulfuric acid, and a soluble salt of a metal of the group comprising iron, nickel and cobalt, and during the passage of electric current through said bath, maintaining the chromic acid contents andthe sulfuric acid radical contents oi the bath, at a ratio of parts chromic acid to 2.0-50 parts sulfuric acid radical.

3. The method of conducting the electrodepositing of chromium. from solutions or chromic acid, which comprises establishing a bath of chromic acid, sulfiu'ic acid, and a soluble salt of a metal of the group comprising iron, nickel and cobalt, controlling the contents oi the solution of dissolved metal of the group-comprising-iron, nickel and cobalt to maintain a content not above 20 grams per litre during the mssage of electriccurrent through said bath maintaining the chromic acid content and the sulfuric acid radical content at a ration! 100 parts chromic acid to 2.0-5.0 parts sulfuric acid radical.

4. The method of conducting the electrode- M positing oi chromium from solutions oi chromic acid, which comprises establishing in, the bath and maintaining therein during electrolysis chromic acid, acid radical and a complex organic chromium compound of the character which results from the introduction into the bath of a substance such as potassium tartrate and maintaining in the bath during electrolysis a ratio of 100 parts chromic acid to 2.0-5.0 parts acid radical.

5. The method which comprises electrodepositing chromium from solutions of chromic acid in a bath containing S04 or an equivalent, reducing said chromic acid, and, simultaneously with the carrying out of the said plating opera-' tion, providing in the bath a non-gaseous pluralelement agent constituted of elements having an afllnity for oxygen as in the case of potassium cyanide, said agent being capable of combining as such with the iron impurities-in the bath, pro- 0 tecting said agent against forming gas by combi-.

nations of one or more 01' its component elements withv oxygen anions resulting from the electrolysis by maintaining in the bath during the electrolysis a second agent which as in the case of potasium tartrate has a greater afflnity for the said oxygen anions than have any of the elements of the plural-element agent and maintaining the chromic acid contents and the sulfuric acid radical contents oi the bath at a ratio of 100 parts chromic acid to 2-5 parts sulfuric acid litre.

finity for oxygen as in the case of potassium cyanide, said agent being capable of combining as such with the metal in the bath and protecting said agent against forming gas by combinations of one or more of its component elements with oxygen anions resulting from the electrolysis by maintaining in the bath during the electrolysis a second agent which as in the case oi. potassium tartrate has a greater afllnity for the said oxygen anions than have any 01 the elements 01' the plural-element agent and, during the passage of electric current through said bath, maintaining the chromic acid content and the sulfuric acid radical content 01 said bath at a'ratio of 100 parts chromic acid to 2.0 to 5.0 parts sulfuric acid radical.

7. The method which comprises conducting the electrodepositing of chromium from solutions of chromic acid in a bath containing 804 or an equivalent, reducing said chromic acid, and,

simultaneously with the carrying out of the said plating operation, providing in the bath a soluble salt of a metal of the group comprlsing'iron,

I nickel, and cobalt, and also a non-gaseous pluralelement agent constituted of elements having an .amnity for oxygen as in the case of potassium cyanide, said agent being capable of combining as such with the metal in the bath and protecting said agent against forming gas by combinations of one or more of its component elements with oxygen anions resulting from the electrolysis by maintaining in the bath during the electrolysis a second agent which as in the case of potassium tartrate has a greater ailinity for the said oxygen anions than-have any of the elements of the plural-element agent and, during the passage 01' electric currentthrough said bath. maintaining the chromic acid content and the sulfuric acid radical content of said bath at a ratio of 100 parts chromic acid to 2.0 to 5.0 parts sulfuric acid radical and also controlling the content of the solution 01' dissolved metal so as to maintain said content at not above 20 grams per osxan 

